Using removable media can be a major risk, and companies that are involved with critical and hazardous environments need to take stringent measures to ensure that industrial control systems (ICSs) and other key components aren’t compromised.

Steven Paul Romero

07/03/2016

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In the wake of news regarding malware infection within a German nuclear power plant (Gallagher, 2016) we should pause to consider the risks posed by removable media use within an industrial control system (ICS) environment. This incident and others like them should serve as reason to outline a practical, high-level approach for managing removable media risk within critical infrastructure environments containing industrial control systems. Assumptions or considerations concerning compliance-focused regulation that prohibits or severely limits removable media use within certain industries won't be made.

The reality is, compliance efforts portray a snapshot in time, and rules are frequently broken. Companies and users need to be more stringent in their approach because the potential damage can be catastrophic. Following these eight steps can help prevent a terrible scenario from occurring.

1) Using removable media is like playing with fire. If companies can avoid using them altogether, they should. This may not be a practical approach for all situations, as is often the case in so-called "air-gapped" environments or those with limited supporting infrastructure for transfer of files and data by another means. Nevertheless, it should be pursued as a goal. At the very least, removable media use should be controlled, limited, and fit-for-purpose. Use policy to communicate requirements regarding the use of removable media and enforce that position with administrative and technical controls.

2) Assess the use of removable media in the environment in order to determine the organization's acceptance of and maturity regarding removable media use. There are many ways to do this, but an effective mechanism is surveys followed by validation of responses. Technical audits will provide some revealing data on past and current insertion of unique removable media on targeted systems. Using the right tools can lend to automated retrieval of this information over the network. Instrumenting the systems to alert on insertion of removable media and enabling centralized logging of these events will allow for a more robust continuous monitoring of the environment.

3) Maintain a pool of approved removable media and implement a dedicated sheep-dip system that can be used to scan the removable media for malware prior to use within the control system environment. Anti-virus (AV) scanners are proven to not be 100% effective, but even 40% to 50% effective is better than no protection at all (Goodin, 2014). Limit the functionality of the sheep-dip system to only scanning removable media for malware, and consider using an advanced threat prevention tool to detonate each executable file on the removable media that you want to use in the control system environment to further increase the chance of detecting maliciousness. Removable media should also be scanned outbound—meaning when the user is finished using the device, it should be scanned again to ensure nothing was picked up in the "secure" zone. While it may sound time-consuming, so is recovering from an incident (not to mention costly).

4) Implement or integrate authorized removable media use into a permit to work system, and leverage the accompanying job safety and hazard analysis to identify hazards and potential outcomes, gain final clarity on the risk, and develop a plan for your work that minimizes removable media use. Perform a last-minute risk assessment immediately prior to starting work to ensure that the process has been thought through and that there aren't gaps in the plan.

5) Provide an alternative to removable media where possible to limit use. Some options may include using an Internet Content Adaptation Protocol (ICAP) managed file transfer solution or an application layer firewall capable of detecting maliciousness within the data flow to scan all files and data being transferred across security zones for malware during transfer. Choosing this alternative will also force companies to account for encrypted traffic flows, and either proxy encrypted sessions during data and file transfer inbound and outbound, or disallow encrypted data transmissions across security zones altogether.

6) Implement compensating controls. Every system capable of supporting anti-virus scanners should have them installed even though their effectiveness is limited. Also strongly consider the use of application whitelisting. Physically block or logically disable USB Mass Storage device use on all nodes to prevent unauthorized or casual use of removable media. For legacy systems that cannot support newer AV engines or patching (e.g. Microsoft Windows XP), a) disable AutoRun/AutoPlay and b) implement application whitelisting, which has proven very effective (although challenging to implement) when done correctly.

7) Effectively manage the organization's third-party engagements such that the expectation for performance in this area is equivalent to the expectations set for direct employees. Enforce performance through contracts, which stipulate clear requirements and liabilities. The U.S. Department of Energy maintains some very resourceful documentation regarding cybersecurity procurement that can help companies navigate these waters (U.S. Department of Energy, 2014).

8) Build a culture of awareness regarding the risks of removable media use with the goal of changing behavior (Lally, van Jaarsveld, Potts & Wardle, 2010). Do this by creating written policies, processes, and procedures around the acceptable use of removable media. Also, implementing a targeted awareness-training program to educate workers on the risks for not following proper procedures. The organization's safety culture should be used as a springboard to accelerate behavioral change and to manage the risk in the environment just like any other process safety or health environment safety (HES) risk.

Steven Paul Romero is energy professional at Chevron. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

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